For a number of years, it has been known that various microorganisms exhibit biological activity so as to be useful to control plant diseases. Although progress has been made in the field of identifying and developing biological pesticides for controlling various plant diseases of agronomic and horticultural importance, most of the pesticides in use are still synthetic compounds. Many of these chemical fungicides are classified as carcinogens by the EPA and are toxic to wildlife and other non-target species. In addition, pathogens may develop resistance to chemical pesticides (Schwinn et al., 1991).
Biological control offers an attractive alternative to synthetic chemical fungicides. Biopesticides (living organisms and the compounds naturally produced by these organisms) can be safer, more biodegradable, and less expensive to develop.
The actinomycetes, including the streptomycetes, are known producers of antifungal metabolites (Lechavalier and Waksman, 1962; Lechavalier, 1988). Several actinomycete-produced antibiotics are routinely used in an agricultural setting such as streptomycin and terramycin for fire blight control.
Streptomycetes have demonstrated both in vitro and in vivo activity against plant pathogens. Axelrood et al. (1996) isolated 298 actinomycetes from Douglas-fir roots. Approximately 30% of these strains demonstrated antifungal activity against Fusarium, Cylindrocarpon, and/or Pythium in vitro. Yuan and Crawford (1995) reported that Streptomyces lydicus WYEC 108 showed both strong in vitro antifungal activity and inhibition of Pythium root rot in pot tests with pea or cotton seed. Reddi and Rao (1971) controlled Pythium damping-off in tomatoes and Fusarium wilt of cotton with Streptomyces ambofaciens. Rhizoctonia root rot was controlled by Streptomyces hygroscopicus var. geldanus (Rothrock and Gofflieb, 1984). These authors reported that the control was dependent on the in situ geldanamycin concentration produced by this strain. The same authors also saw protection of soybeans from Phytophthora megasperma var. sojae by Streptomyces herbaricolor and Streptomyces coeruleofuscus (1984). Chamberlain and Crawford (1999) saw in vitro and in vivo antagonism of turfgrass fungal pathogens by S. hygroscopicus strain YCED9. Crawford (1996) patented the use of this strain to control plant pathogens in U.S. Pat. No. 5,527,526. Suh (1998) patented 2 Streptomyces sp. that were active against Rhizoctonia solani and Phytophthora capsici. A Streptomyces griseoviridis product against Fusarium spp. and other soil pathogens is on the market as Mycostop™.